Oscillation generator



sept. 14, 1937..

L. H. LYNN 0S CILLATION GENERATOR Filed Aug. 1, 1935 FLJ.

ITM/Shim, Lambert H. L, gnh,

s Ahto'rheg.

UNITED ASTATI-:s

amsn PATENT ori-ic oscnins'rron manon Lambert ll. Lynn. Schenectady, N. Y., 'aligner to General Electric Oompany, a corporation of New York applicano; sus 1, 193s. serai No. scm

including an electron discharge tube having a frequency determining circuit connected thereto.

A My invention is particularly. applicable for use in connection with electron discharge oscillators Iwherein it is desired that the voutput frequency be maintained substantially constant under all operating conditions.

Ihe conventional electron discharge oscillator usually includes a thermionic device having a control electrode, a cathode and an anode, t0- gether with a tunable frequency-determining circuit connected to supply a control potential to l5 the control electrode thereby to maintain the oscillator in a stable operating condition. Such oscillators are designed to generate electrical oscillations of a predetermined frequency and in many applications any deviation from the normal operating frequency is highly objectionable. It has been found that temperature variations in the elements of apparatusl of this type cause a wide drift', or change in the frequency of the generated oscillations.

' 25 It is an object of my invention to provide a new and improved method of and means for minimizing frequency driftin the output frequency of an electron discharge oscillator. v

It has been found that frequency drift in the 30 output frequency of an oscillator of the aboveindicated type is primarily attributable to temperature variations within the electron discharge device of the oscillator and to temperature variations of the elements forming the frequency '35 determining circuit.

It is a further object of my invention to provide in combination with an electron ,discharge oscillator, means for varying the tuning of the frequency determining circuit in accordance with 40 the temperature within the discharge device to lator circuit and the second factor which influ-v oo .ences the'temperature within the control tube is the magnitude oi' the average current owing in the anode circuit of the oscillator.

It is, therefore, an additional object of my invention to provide in Vcombination with an oscillation generator including a frequency de- 5 termining circuit connected to an electron discharge control tube having a cathode, means for varying the tuning of the frequency determining circuit in accordance with the temperature of the cathodeof the electron discharge tube thereby to 1o minimize frequency drift in the output frequency ofthe generator.

A still further object of my invention is to provide in apparatus of the above-indicated character means for varying the tuning of the frel5 quency determining circuit in accordance with variations in the magnitude of the average current flowing in the anode circuit of the oscillator.

In accordance with my invention compensation for frequency drift caused by temperature varia- 20 tions within the tube is obtained by providing an auxiliary variable condenser connected in shunt with av portion, or all, of the frequency determining circuit, which may be actuated to vary the tuning of the circuit in accordance with the amount of. energy supplied to the cathode to heat the same and in accordance with the mag-` nitude of current in the anode circuit.

In' the specinc embodiment of my invention illustrated and described hereinafter the variable condenser comprises a stationary electrode and a movable electrode mounted on the thermostatic element. The thermostatic elementV is energized to vary the capacity between the stationary and movable electrodes in Vaccordance with the temperature within the oscillator tube by a pair of heating resistances connected respectively in the denser connected in shunt with the frequency determining circuit of the oscillator and including m a movable electrode which is mounted on a thermostaticelement, the latter element being energiaed by a pair of resistances connected respectively in the anode and cathode circuits of the oscillator.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and the method of operation, together with further objects and advantages thereof, may best be understood by reference to the following specification taken in connection with the accompanying drawing, in which Fig. l illustrates a circuit arrangement having my invention embodied therein and which operates in accordance with the method to be described hereinafter; Fig. 2 illustrates the operating characteristics of the circuit shown in Fig. l; Fig. 3 is a side view of an element of the circuit shown in Fig. l, and Fig. 4 is a detailed view of a modification of the element shown in Fig. 3.

Referring to Fig. 1 of the drawing I have shown my invention as applied to an electron discharge tube oscillator of a type well known in the art.

The oscillator comprises an electron discharge tube I having an anode 2, a cathode 3, and a control grid 4. The tube -I is shown as being of the directly heated cathode type, although it will be understood by those skilled in the art that a tube of the indirectly heated cathode type maybe 'inductance 8 shunted by a condenser 9. I terminal of the tuned circuit l is connected to the lemployed with but a slight modification of the circuit arrangement shown. 'I'he cathode 3 of the tube I is energized from a suitable source of low voltage shown in the form of a battery 5, which control grid 4 of the tube I through a circuit lead which includes a grid condenser I0. A grid leak resistor II is connected between cathode 3 and grid 4. The anode circuit of the oscillator includes the lower portion of the inductance 8 and afsuitable source of high voltage I2 shunted by a high frequency by-pass condenser I3. The anode circuit is connected between the anode 2 and the cathode 3 of the discharge device I in the manner illustrated and a portion of the oscillatory energy developed in this circuit may be conducted to a utilizing circuit (not shown) by means of the circuit leads I 4.

In the operation of the above-described circuit and with the cathode 3 of the device I heated to its normal electron emission temperature, if a voltage be impressed on the tuned circuit 1, electrical oscillations are produced therein having a frequency determined by impedance constants-of the circuit. A portion of the energy developed by such oscillations is impressed on the control grid 4 of the tube I through the circuit lead including the grid condenser Ill and functions to maintain the circuit in a stable oscillatory condition. The operation of the oscillator described above is Well understood in the art and a further -explanation thereof is therefore deemed to be unnecessary.

In certain applications it is desirable to employ a control oscillator which is capable of producing electrical oscillations of substantially constant frequency under all operating conditions. One such example is that of a radio transmitter wherein a master oscillator, which may be of the type described above, is used to control the carrier frequency generated by the transmitter. It has been found that drift in the output frequency of electron discharge tube oscillators is largely attributable to temperature variations within the oscillator control tube and to temperature variations of the elements of the frequency determining circuit. Variations in the temperature within the tube cause warping of the elements of the tube thereby resulting in a change in the interelectrode capacity between the tube elements and an attendant change in the resonant frequency of the frequency determining circuit connected -between the tube elements. In like manner variations in the temperature of the frequency determining elements produce variations in the impedance characteristics thereof whereby the natural frequency of the circuit in which they are connected is varied.

In accordance with my invention frequency drift in the output frequency of apparatus of the above-described type is minimized by the provision of means responsive to variations in the temperature existing within the tube, and to variations in the ambient temperature of the media surrounding the frequency determining circuit elements for varying the tuning of the frequency determining circuit. 'This means is shown as including a small auxiliary condenser I5 having lits two electrodes I6 and Il connected across a portion of the frequency determining circuit, one of the electrodes being movable with respect to the other electrode inresponse to the movement of a thermostatic element I8 which is energized by a pair of heating resistances I9 and 20 connected respectively in the cathode and anode circuits of the tube I.

The operation of the above-described arrangement to produce the desired compensating action is as follows:-During the starting period of the apparatus and following a closure of the circuit interrupting device 6 to energize the cathode heater circuit, the cathode 3 of the tube VI starts to heat up to its normal electron emission temperature. The heat generated by the cathode raises the temperature of the media within the tube and the temperature of the elements therein. This increase in the temperature of the ele' change in the capacity of the condenser I5 tends to compensate for the variation in capacity caused by the warping of the tube elements and by a proper calibration of the movement of the electrodes of condenser I5, the compensating ac-V tion may be adjusted to prevent any appreciable change in the frequency of the oscillations generated by the oscillator during thestarting period.

Following an initiation of the operation of the oscillator, anode current flows between the anode 2 and the cathode 3 of the tube and through the anode circuit of the oscillator. This current may be varied in magnitude in several different ways. For example, if the oscillator be embodied in a radio transmitting system andthe control oscillator is keyed to produce coded impulses of control current, the anode current of the oscillator will fluctuate between zero and a normal value during each keying operation. Such fluctuations in the anode current tend to vary the mean value of heat energy developed within the tube by the anode current and are accordingly attended with variations in the temperature within the tube which cause undesired changes in the inter-'elec'- trode capacity between the tube elements. The effect of such capacity changes in producing u'ndesired variations in the output frequency of the oscillator has been previously described. In accordance with my invention the above-noted undesired temperature variations are compensated 'for by energization of the heating resistance II connected in. the anode circuit of the oscillator to produce aiiexure of the bi-metallic element II which is proportional to the average current nowing in the anode circuit. Thus, `assuming an increase in the average anode current such that a decrease in the output frequency of the oscillator is produced, heat generated by theincreased current through the resistance II causes the element II to be flexed further to the left todecrease the `capacitance value of the condenser II and thereby change the tuningof the resonant circuit 1 to increase the output frequency of the oscillator.

AIt should be noted that arr-appreciable time lag occurs between a change in the temperature within the tube and a warping of the tube elements to conform to such change. This of course,

means that instantaneous variations in the anode current do not produce corresponding instantaneous changes in the inter-electrode capacity between the tube elements. However, if the anode current is being varied over wide. ranges, as for example. by keying the oscillatonxthe amount of heat energy dissipated within the tube assumes a mean value considerably lower than that which would obtain if the anode current were maintained constant at a normalf: operating value.

This means' that the average temperature is lower and the displacement between the elements is different. It will be seen, therefore. that changes in the inter-electrode capacities between the tube elements are produced by changes in the average value of the anode current as contrasted to instantaneous current variations. The operation of the condenser II is such that changes in the capacity thereof are lnraccordance with, and are proportional to, the average value of the anode current. Thus, as the instantaneous value of anode mrrent fluctuates, the heat energy devel- Oped by the resistance II is varied in accordance with the wattage input to the resistance, which is, in turn, determined by the -average current flowing through the resistance.- The temperature increase of the element II dueto the heat developed by resistance II is proportional to the heat energy output of this resistance and. since the amount of ilexure of the element attributable to the heat produced by resistance II is proportional to this temperature increase, it will be seen erated by the resistance II produces a iiexure 'which varies in accordance with the average current flowing in the anode circuit of the oscillator. 'Ihe total heating effect` produced by the .amasar momentanea n and n is auch um the siement II is nexed in accordance with the temperature within the tube I and the capacitance of the condenser Il is accordingly varied ,in accordance withthetemperature within the tube i.

'l'he thermostatically actuated auxiliary condenser Il may be designed to perform an added associated resistances II and II will be substantially the same as that of the media surrounding the elements I and I. Thus. as this ambient temperature rises the overall temperature of the element II is raised irrespective of the heat energy being supplied by the resistances II and II. Ihe element II is flexed in response to such an elevation in the temperature thereof thereby producing a variation in the capacitance of condenser II which tends to compensate for changes in the natural frequency of the` resonantl circuit 'I resulting from the variations in temperature of the media surrounding the elements I and I.

The effect of employing my improved compensating arrangement in connection with an oscillator of the type describedabove is best lllustrated by reference to Fig. 2 wherein I have shown curves plotted from data taken from tests run on a spciilc installation. In this figure variations in the frequency output of an oscillator from its normal operating frequency are plotted against time; the curve A being plotted from data taken on an uncompensated oscillator and the curve B being plotted from data taken during the operation of the same oscillator when equipped with my improved compensating arrangement. From these curves it will readily be seen that the deviation in the frequency of the output of the oscillator equipped with a compensating device arranged in accordance with my invention is at all times substantially less than the deviation in frequency of an uncompensated oscillator.

It will be observed from the curves of Fig. 2 that during the starting period of the apparatus. corresponding to a time interval of approximately four minutes for the equipment tested, the compensating device over-compensates for the frequency change caused by the warping of the tube elements. Over-compensation would, of course, be unnecessary if compensation were emcient from the moment of initial energization of the oscillation generator throughout the entire operation period. When the mass of the thermostatic element II is'less than the mass of the elements in discharge device I the tem-L perature of the former rises more rapidly than that of the latter and hence over-compensation takes place. If the oscillation generator be properly compensated for normal operation, a large amount of over-compensation occurs during the initial energization period. If the oscillation generator be properly compensated for the initial energization period under-compensation occurs to a marked degree during normal operation. It is, therefore, `desirable to strike some compromise between there two extremes in Order that the percent deviation over theentire period of f operation -be minimized as much as possible.

Referring to Fig. 3 of the drawing I have illustrated an auxiliary condenser which is particularly adaptable for discharging the compensating functions described in the preceding paragraphs. The condenser is shown as comprising l a pair of electrodes I6 and I1, the latter electrode being movable with respect to the former in response to the flexure of the bi-metallic element I8 upon which it is mounted. The element I8 is mountd on a supporting base 2| which is shown as being mounted on a panel 22 by means of a screw 23. In order to permit the condenser I 5 to be readily adjusted to secure the correct initial adjustment of the frequency determining circuit 1, the electrode `IS is adjustably mounted adjacent the movable electrode I1 on a supporting member 24 which is flxedly mounted on lthe base 20 by means of a screw 25. 'Ihe adjustable connection between the supporting member 24 and the electrode I6 comprises a threaded stud 26 extending from the electrode I6 and a. pair of lnuts 21 and 28 threaded on the stud 25 and positioned on either side of the support 24. The heating coils I9 and 20 are indicated as being located on either side of the bi-metallic element I1 in close physical proximity thereto. Preferably the heating coils are insulated from the element I8 by thin/mica strips 29 mounted on the member 2I and extending on either side of the element I8. If desired, the entire assembly may be surrounded with a layer of asbestos or other heat insulating material to insure a good transfer of the generated heat to the element I8.

In the construction and operation of a condenser suited to the purpose described above, it has been found that stray mechanical vibrations produce vibrations of the movable element I1 which cause frequency pulsations in the output frequency of the oscillator. In order to eliminate this undesired vibration of the electrode I1 a second bi-metallic element 38 may b e positioned directly adjacent the element I8, and having a surface thereof in frictional contact with a surface of the element I8. The element 30 should be biased against the element I8 to insure a good frctional engagement between the surfaces of the two elements. In addition, the-temperature ilexure characteristics of the two elements I8 and 30 should be identical and they should be so arranged that the free ends thereof move in the same direction under like changes in temperature. With this composite structure, mechanical vibrations which tend to produce vibrations of the electrode I1, are substantially prevented by the damping effect created by the frictional engagement between the two elements I8 and 30. Since the two elements are identical in their flexure characteristics, the compensating action is not interfered with.

In designing elements I1 and 30 of sufficient sensitivity to produce the desired compensating action with a low power input to the resistances I9 and 20 it has been'found to -be necessary to-use elements which are over sensitive to changes in the ambient temperature of the media surroundving-the element.v In-other words, such ambient temperature changes produce an over-compensation and cause an undesired frequency variation in the output frequency of the oscillator. This-undesired feature can, of course, be obviated by increasing the power input to the resistances I9 and 20 and making the elements I9 and 30 less sensitive to temperature variations.

electrode I6 on a thermostatic element 3i in the manner illustrated in Fig.,4. The element 3i should possess a temperature flexure characteristic such that movements of the free end thereof for predetermined temperature variations are substantially less than the movements of the electrode I1 for like temperature variations. Obviously the element 3i should be arranged in such a way that the free end thereof moves in the same direction as the elements I8 and 30 in response to like temperature changes. In this manner the over-compensating effect caused by the high sensitivity of the elements I8 and 29 is neutralized by a movement of the electrode I6 in response to exure of the element 30. By a proper selection of the length and physical properties of the element 30 the device may be calibrated so that no over-compensation occurs due to the high sensitivity of the elements I8 and 29. It will of course be understood that a fourth bi-metallic element may be used to prevent vibration of the element 3i in the same manner that the element 30 prevents vibration of the element I8.

It has been found from experienceV that frequency drift in the output frequency of an oscillator equipped with my improved compensating arrangement is substantially reduced, and that once the compensating device has been properly calibrated it operates faithfully to perform the desired compensating action under widely divergent conditions of oscillator operation.

Although I have described my improved compensating device-as being used in connection with an electron discharge oscillator, it will of course be understood by' those skilled in the art that it may have utility in connection with other types of networks in which' a determining circuit is connected to an electron discharge device. While I have described my improved compensating arrangement as including means for compensating for variations in the average value of the anode many modifications in the structure may be made,

and I contemplate by the appended claims to cover all such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, an electron discharge device including a cathode and an anode, a tunable frequency determining circuit connected to said device, an auxiliary variable condenser having relatively movable electrodes, a thermostatic element for producing relative movement between said electrodes, and means including a pair of resistances connected respectively in circuit with said cathode and in circuit with said anode for energizing said element to vary the tuning of said frequency determining circuit in accordance with variations in the temperature within said device.

2. In an oscillatory circuit of the class having an electron discharge device, a power supply circuit therefor, and a frequency determining circuit connected thereto, said discharge device including an anode and a cathode, the combination withv said discharge device and said frequency determining circuit of an auxiliary variable condenser having relatively movable electrodes connected in said frequency determining circuit, a thermostatic element for producing relative movement between said electrodes, said thermostatic element being positioned in proximity to the elements of said frequency determining circuit to compensate for variations in ambient temperature, means responsive to variations in the cathode heating current of said discharge device for producing a false ambient about said thermostatic element, and means responsive to variations in average plate current flowing through said discharge device for producing a false ambient about said thermostatic element.

3. In combination, an electron discharge device including a cathode, an anode element, and a grid element, a source of heating current operatively connected to heat said cathode, a voltage source operatively connected to supply a voltage to said anode, a tunable frequency determining circuit operatively comected to said cathode and at least one of said elements, an auxiliary variable condenser separate from said discharge device and having relatively movable electrodes. a thermostatic element for producing relative movement between said electrodes, and means for energizing said thermostatic element to vary the tuning o f said frequency determining circuit in accordance with variations in the temperature within said device, said means including means connected to said cathode -heating source and means connected in series with said anode and said anode voltage source.

4. In combination, an electron discharge device including a cathode,- an anode element, and a. grid element, a source of heating current operatively connected to heat said cathode, a tunable frequency determining circuit operatively connected to said cathode and at least one of said elements, an auxiliary variable condenser separate from said discharge device and having relatively movable electrodes, a thermostatic element for producing relative movement between said electrodes, and means for energizing constantly during the initial heating of said cathode to the electron emission temperature said thermostatic element by current supplied from said cathode heating source, said electrodes, said thermostatic element, and said means being so arranged that the tuning of said circuit is changed to compensate for change in the inter-electrode capacity of said discharge device caused by warping of electrodes therein when the temperature of said cathode is raised from cold condition to said electron emission temperature.

LAMBERT H. LYNN. 

